1. Field of the Invention
The present invention relates to a method for recharging raw material polycrystalline silicon, particularly to a method for recharging raw material polycrystalline silicon intended for re-supplying chunks of raw material polycrystalline silicon into a crucible in a method for producing a silicon single crystal ingot (hereinafter, “ingot”) using the Czochralski method (hereinafter “CZ method”).
2. Description of the Related Art
A semiconductor substrate used for producing a semiconductor device is made of a single crystal silicon wafer, and the CZ method is widely used for growing silicon single crystal ingots. In the CZ method, a crucible is filled with polycrystalline silicon, and this polycrystalline silicon is melted to obtain a silicon melt. Next, a seed crystal is brought into contact with this silicon melt and an ingot is grown by pulling up the seed crystal.
It is difficult to reuse a crucible that has already been used. Therefore, in order to reduce the costs of producing an ingot, a multi-pulling technique is conventionally used wherein the polycrystalline silicon which is the raw material, is re-supplied into the crucible to compensate for the silicon melt which has decreased because of the pulling-up of the ingot, and ingots are repeatedly pulled up without exchanging the crucible.
For example, a method for recharging polycrystalline silicon by means of a cylindrical recharge tube is disclosed in WO 02/068732. This recharge tube comprises a cylindrical tube and a detachable conical valve at the lower end of this tube. In the recharge method using this recharge tube, while the tube is blocked by the conical valve, solid-state polycrystalline silicon is filled inside the tube, the recharge tube is disposed above the crucible after the ingot has been removed, the conical valve is detached to open the lower end of the tube, and the polycrystalline silicon is supplied into the crucible from the lower end of the tube.
According to this conventional recharge method, the polycrystalline silicon is dropped from the recharge tube into the silicon melt in the crucible, and therefore the silicon melt in the crucible is splashed. If the silicon melt is splashed out of the crucible, it causes damage to the ingot production apparatus, the growth process of the ingot must be stopped, and serious damage is caused to the growth process of the ingot. Further, impurities of a predetermined percentage are mixed in the silicon melt, and the splash of the silicon melt changes the impurity concentration of the silicon melt in the crucible. As a result, an ingot without the desired quality is grown, causing a degradation in quality.
Therefore, methods for preventing the silicon melt in the crucible from being splashed at the time of recharging have been conventionally disclosed.
For example, the following method is disclosed in WO 02/068732. Before the polycrystalline silicon is dropped, the output of a heater is lowered to solidify the surface of the silicon melt in the crucible. Thereafter, the output of the heater is increased and the raw material solid-state polycrystalline silicon is dropped from the recharge tube.
However, in the conventional recharge method, when large chunks of polycrystalline silicon are dropped, the impact of the dropped polycrystalline silicon cannot be cushioned. Then, the dropped polycrystalline silicon breaks through the surface of the solidified silicon melt, and the silicon melt in the crucible may be splashed, possibly stopping the production of ingots. For this reason, in the conventional recharge method, large chunks of polycrystalline silicon cannot be dropped. Large chunks of raw material polycrystalline silicon have the advantage that they can be produced at low cost and that therefore the production costs can be reduced. Further, in the case when large chunks of raw material polycrystalline silicon are used, the total surface area of the raw material polycrystalline silicon is smaller at the same weight than in the case when small chunks of polycrystalline silicon are used. As a result, SiO2 on the surface of the polycrystalline silicon, impurities such as metals, and the mixing-in of atmospheric gases, etc. can be reduced. Thus, in the case of using the large chunks of the raw material polycrystalline silicon, in the grown ingot, the decline of the dislocation free rate and the quality because of contamination by impurities, formation of pinholes, etc. can be restricted.
Moreover, in the conventional recharge method, the surface of the silicon melt in the crucible is solidified. Therefore, the volume is expanded at the surface of the silicon melt and a force compressing the inner wall of the crucible is generated. This force is likely to rupture the crucible.